Apercentage point orpercent point is the description for thearithmetic difference between twopercentages. For example, moving up from 40 percent to 44 percent is an increase of 4 percentage points (although it is a 10-percent increase in the quantity being measured, if the total amount remains the same).[1] In written text, the unit (the percentage point) is usually either written out,[2] or abbreviated aspp,p.p., or%pt. to avoid confusion with percentage increase or decrease in the actual quantity. After the first occurrence, some writers abbreviate by using just "point" or "points".
Consider the following hypothetical example: In 1980, 50 percent of the population smoked, and in 1990 only 40 percent of the population smoked. One can thus say that from 1980 to 1990, the prevalence of smoking decreased by 10percentage points (or by 10 percent of the population) or by20 percent when talking about smokers only – percentages indicate proportionate part of a total.
Percentage-point differences are one way to express a risk orprobability. Consider a drug that cures a given disease in 70 percent of all cases, while without the drug, the disease heals spontaneously in only 50 percent of cases. The drug reduces absolute risk by 20 percentage points. Alternatives may be more meaningful to consumers of statistics, such as thereciprocal, also known as thenumber needed to treat (NNT). In this case, the reciprocal transform of the percentage-point difference would be 1/(20pp) = 1/0.20 = 5. Thus if 5 patients are treated with the drug, one could expect to cure one more patient than would have gotten well without receiving the treatment.
For measurements involving percentages as a unit, such as, growth,yield, orejection fraction,statistical deviations and relateddescriptive statistics, including thestandard deviation androot-mean-square error, the result should be expressed in units of percentage points instead of percentage.[citation needed] Mistakenly using percentage as the unit for the standard deviation is confusing, since percentage is also used as a unit for therelative standard deviation, i.e. standard deviation divided by average value (coefficient of variation).